GB2071312A

GB2071312A - Method and apparatus for analysis of a multiphase fluid containing liquid and a free gas

Info

Publication number: GB2071312A
Application number: GB8102723A
Authority: GB
Original assignee: Texaco Development Corp
Current assignee: Texaco Development Corp
Priority date: 1980-03-06
Filing date: 1981-01-29
Publication date: 1981-09-16
Also published as: GB2071312B; FR2477713A1; NO810657L; JPS56130643A; US4365154A; NL8100948A; FR2477713B1; DE3107327A1; CA1148670A

Description

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GB 2 071 312 A 1

SPECIFICATION

Detection of impurities in a fluid containing free gas using nuclear techniques

The present invention relates to nuclear techniques for detecting impurities such as salt water and sulfur in petroleum refining and producing operations.

5 Our U.S. Patent No. 4,209,695 relates to a nuclear technique for measuring the chlorine and sulfur content of a flowing stream of fluid. However, the technique of that patent required that any free gas in the fluid be homogeneously mixed. Otherwise, any free gas in the stream of fluid introduced errors in the chlorine (and thus salt content) and sulfur measurements by varying the relative shielding properties of the fluid. Thus, for accurate results apparatus of the type in that patent was typically at the 10 output of a gas-oil separator. There are, however, other locations in petroleum producing or refining operations where gas is present in the fluid and it is desirable to monitor the salt content of the fluid.

Our U.S. Patent No. 4,200,789 relates to a technique for measuring oil and water cuts of a multiphase flowstream. The flowstream was bombarded with neutrons and high energy gamma rays resulting from the capture of thermal neutrons was detected. The spectra of the detected gamma rays 1 5 were analyzed and the gamma ray counts of the element sulfur and the element chlorine determined. Since the gamma ray spectra of the element hydrogen was not needed or used, the effects of gas in the stream on oil and water cut measurements were eliminated.

Briefly, the present invention relates to a new and improved method and apparatus for determining the presence of chlorine in a fluid containing free gas in a petroleum conduit or the like. 20 The fluid is bombarded with fast neutrons from a neutron source which are slowed down and thereafter engage in thermal neutron capture reactions with materials in the fluid, giving rise to thermal neutron capture gamma rays. The energy spectra of the thermal neutron capture gamma rays for the elements hydrogen and chlorine are obtained, from which a measure of the relative concentration of hydrogen and chlorine in the fluid may be ascertained. From the counting rate for the thermal neutron 25 gamma ray spectra for the element hydrogen, a measure of the hydrogen index (HI) of the fluid is obtained. The hydrogen index of the fluid and the ratio of the relative concentration or chlorine to hydrogen are used to obtain a measure of the presence of chlorine or salt water in the fluid.

In further aspects-of the present invention, the concentration of sulfur may also be determined simultaneously with the concentrations of hydrogen and chlorine. Also, since the line temperature, line 30 pressure, hydrogen index of the liquid phase and hydrogen index of the gas phase are usually available or can be monitored, the gas/liquid ratio of the fluid can be computed.

Fig. 1 is a schematic block diagram of apparatus according to the present invention;

Fig. 2A and 2B are graphical illustrations of typical thermal neutron capture gamma ray spectra for crude oil;

35 Fig. 3 is a graphical illustration showing the count rate ratio in chlorine detection energy windows to that of hydrogen energy windows as a function of hydrogen index;

Fig. 4 is a graphical illustration of counting rate in hydrogen energy windows as a function of hydrogen index;

Fig. 5 is a graphical illustration showing hydrogen index and relative presence of chlorine as 40 functions of the count rate ratio in chlorine detection energy windows to that of hydrogen energy windows and of the hydrogen count rate;

Fig. 6 is a graphical illustration of percent standard deviation of results of the present invention;

Fig. 7 is a graphical illustration showing the count rate ratio in chlorine detection energy windows to that of hydrogen energy windows as a function of hydrogen count rate for an enlarged hydrogen 45 energy window of Fig. 2B;

Fig. 8 is a graphical illustration showing hydrogen index and relative presence of sulfur as functions of the count rate ratios of sulfur and chlorine energy windows to that of the hydrogen energy windows and of the hydrogen count rate; and

Fig. 9 is a graphical illustration showing hydrogen index and relative presence of sulfur as 50 functions of the count rate ratio in chlorine detection energy windows to that of hydrogen energy windows and of the hydrogen count rate for various non-homogeneous fluids.

Fig. 1 shows an apparatus A according to the present invention with a neutron source S and a detector D mounted along a common axis mounted adjacent each other on an outer surface of a crude oil flow line 10. The source S and detector D may also, if desired, be mounted within the line 10 along a 55 common axis. The source S shown is a Am—B neutron source emitting 1.33 x 107 neutrons per second, although it should be understood that a different source material such as actinium-beryllium, californium252 or americium-beryllium could be used, if desired. The source S is preferably surrounded by a suitable shielding material 12, such as graphite, which thermalizes but does not capture neutrons in order to increase the thermal neutron flux for capture within the fluid of interest. The intervening 60 space 13 between the source S and detector D is also filled with such a material. The detector D is preferably a 2" x 4" Nal(TI) cylindrical crystal coupled to a photomultipliertube T.

It is preferable to enclose the detector D within a sleeve of durable material 14 coated with a coating material of high thermal neutron capture cross-section, such as boron. This is especially true if iron cannot be eliminated in the fabrication in the section of the pipe 10 with which the apparatus A is

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GB 2 071 312 A 2

mounted. When the line 10 is made from steel, it is lined internally with boron or other suitable material in the vicinity of the source S and detector D. This boron coating material reduces the possibility of additional "background" radiation. Boron (boron carbide mixed with epoxy resin) is especially suited for this application since it has a large thermal neutron capture cross section (a = 775 barns) and a capture 5 reaction which produces no radiation above 0.5 MeV to interfere with the chlorine measurement to be set forth.

The detector D produces scintillations or discrete flashes of light whenever gamma rays pass therethrough, while the photomultipliertubeT generates in response to each such scintillation a voltage pulse proportional to the intensity of the scintillation. A high voltage power supply 15 is provided for the 10 photomultiplier tube T. A conventional preamplifier circuit 16 receives operating power from a B"1" power supply 17 and amplifies the pulses from the photomultiplier tube T and furnishes the amplifier pulses to a further amplifier stage 18.

The output pulses from the amplifier 18 are furnished to a gain stabilizer circuit 20 which is calibrated to respond to the energy level of a selected reference peak in the gamma ray energy 15- spectrum, such as the pronounced 2.23 MeV energy peak 22 of hydrogen (Figs. 2A and 2B). It should be understood, however, that other gamma ray energy peaks, a peak generated from the output of a light emitting diode positioned within the detector crystal D, or a mercury pulser may be used for gain stabilization, if desired. The gain stabilizer circuit 20 is an automatic gain control circuit which responds to energy level of pulses at the calibrated peak level and adjusts the gain of all energy level pulses to 20 compensate for gain shift or variations in tube T and other circuitry of the apparatus of the present invention due to power supply voltage fluctuations and/or temperature effects.

The output pulses from gain stabilizer circuit 20 are supplied to a pulse height or multi-channel analyzer 24. The pulse height analyzer 24 may be of conventional design as known in the art and having, for example, two or more channels or energy divisions corresponding to quantizations or energy 25 ranges of the pulse heights of the input pulses, if desired. The pulse height analyzer 24 functions to sort and accumulate a running total of the incoming pulses into a plurality of storage locations or channels based on the height of the incoming pulses which, it will be recalled, is directly related to the energy of the gamma rays causing the pulse. The output of the pulse height analyzer 24 in the case of the present invention consists of count pulses occurring in each of two energy ranges or windows (as depicted in 30 Fig. 2B) or, alternatively three energy ranges or windows (as depicted in Fig. 2A). It should also be understood that two appropriately biased single channel analyzers may be used in place of the multichannel analyzer 24, if desired.

The output from the pulse height analyzer 24 may be stored on a suitable memory device for subsequent processing, or alternatively, is supplied directly over an appropriate number of lines to a 35 computer 26, which obtains from the number of chlorine counts, the number of hydrogen counts and the length of time for such counts, the water cut of the fluid in the line 10, in a manner to be set forth. Further, the computer 24 may also determine from the output of analyzer 26 other measurements regarding the fluid in line 14, as will be set forth. The results of such computations may be stored or displayed, as desired with a recorder 28 or other suitable display device.

40 Fig. 2A shows a typical capture gamma ray spectrum 32 recorded using the equipment of Fig. 1 for a stream of crude oil containing free gas as well as small amounts of chlorine. The intense peak of 2.23 MeV of hydrogen indicated by reference numeral 22 results from the capture of thermal neutrons by hydrogen in the crude oil and may be used, as set forth above, as an energy reference peak by the gain stabilizer circuit of Fig. 1. Fig. 2B also shows the energy settings of the multi-channel analyzer 24. 45 The first setting, identified as "Window I", extends from 5.00 to 8.0 MeV and includes photoelectric and escape peaks from the 7.79, 7.42, 6.64 and 6.11 MeV radiation from the CI3S (n, y) CI36 reaction as well as 5.42 MeV sulfur capture peak and the less intense 7.78, 7.42, 7.19, 6.64 and 5.97 MeV peaks from sulfur. The second setting, identified as "Window 2", extends from 2.05 to 2.50 MeV and includes the 2.23 MeV hydrogen capture peak identified by reference numeral 22.

50 Relatively small concentrations of salt water in crude oil can often cause major problems in the crude oil refining process. The present invention relates to the detection in a flowing multiphase fluid crude oil stream, or other petroleum conduit, of the amount of salt in the fluid while eliminating the effects of gas, such as free gas, in the fluid on the measurements.

In the U.S. Patents referenced above, the space between the neutron source and the gamma ray 55 detector was filled with the fluid of interest. Such a source-detector geometry produced the maximum salt detection sensitivity and introduced no problems in the accuracy of the measurement so long as the fluid contained no free gas. In addition, the fluid served as a constant density shield between the detector and the direct neutron flux from the source. Although the fluid did not completely shield the detector, the source-induced background remained constant.

60 If, however, free gas is present in the fluid in pipe 10 the shielding properties of the fluid are decreased depending upon the fraction and homogeneity of the free gas-liquid mixture. This varies the source-induced background level which, in turn, significantly degrades the accuracy of the liquid phase salt content measurement.

According to the present invention, the adverse effects of the variable neutron-induced 65 background can be minimized, though, by filling the space between source S and detector D with fixed

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GB 2 071 312 A

graphite shielding material rather than the fluid of interest. Some precision may be lost with this source-detector geometry; however, in the present of free gas, the accuracy of the salt content measurement is superior to that obtained with the source and detector on opposite sides of the flow pipe 10.

In a test of the present invention, a section of fiberglass pipe 10 was filled with 45,850 cm3 of tap 5 water which contained approximately 25 pounds per thousand barrels (PTB) of NaCI. A series of gamma ray spectra were recorded so that the observed as well as statistical standard deviation could be computed. RC|, the ratio of count rate Cc, recorded in the energy region 5.75 to 8.0 MeV (chlorine window) (Fig. 2A) to the count rate CH recorded in the energy region 2.05 to 2.50 MeV (hydrogen window), was computed for each spectrum. Known amounts of chlorine, MC|, in the form of NaCI were 10 added to the water in increments. Following each addition, the above counting sequence was repeated.

Void spaces representing free gas were introduced within the pipe 10, having 10f inch diameter, by displacing a portion of the liquid with thin walled, air filled acrylic resin tubes mounted at selected locations within the cross-sectional area of the pipe 10 and extending from the source S to the detector D. Acrylic resin tubes were used because the neutron properties of some acrylic resin, such as that sold 1 5 under the Du Pont trademark Lucite, are almost identical to those of fresh water. Groups of 21,40 and 49 tubes were arranged in regular arrays to simulate homogeneous fluids with hydrogen indices, HI = 0.83,0.68 and 0.61, respectively. The gamma ray counting sequence was repeated at each hydrogen index using water with various salinities.

Hydrogen index, HI, is proportional to the quantity of hydrogen per unit volume of fluid with the 20 hydrogen index of fresh water taken as unity. HI is related to the gas phase fraction Vg by the relation:

(HI)L — HI

Vg= (1)

(Hl)L— (Hl)g where (HI)L and (Hl)g are the hydrogen indices of the liquid and gas phases, respectively.

Fluids with HI = 0.47, 0.39, 0.20 and 0.11 were simulated by arranging water filled tubes in regular arrays of 49, 40,21 and 12, respectively, with the pipe 10 empty. Again, the counting sequence 25 was repeated for each array using water with various salinities.

Figure 3 shows a plot of the measured quantity Rc(/ as a function of HI for measures of the relative presence of chlorine, Mc,, expressed in PTB. Each data point represents a total count time of 25 minutes with the number in parenthesis indicating Mc,, the salinity of the liquid phase in PTB. The constant salinity curves were obtained by least-squares fitting all available data. It can be seen from the spread of 30 these curves that the precision to which Mc, can be measured increases as the HI of the fluid increases.

Fig. 4 shows a plot of observed hydrogen window count rate CH as a function of HI. Based on a measure CH in window 2 (Fig. 2A) the hydrogen index HI for the multiphase fluid under test may be readily determined. It is to be noted that the relationship between hydrogen count CH and hydrogen index HI is nearly linear as can be theoretically predicted.

35 Fig. 5 shows a plot of Rc, as a function of CH and is a compilation of information presented in Figs. 3 and 4. The numbers in parenthesis indicate the values of Mc, and HI, respectively, which correspond to each data point. It should be noted that both Rc, and CH are measured quantities in the apparatus A. These quantities, along with a chart of the form of Fig. 5, can be used to determine Mc, (and HI) for any unknown fluid. As an example, recorded values of CH = 225,000 counts/5 minutes and 40 Rc, = 0.065 indicate a fluid with a liquid phase salinity Mc, = 500 PTB and a hydrogen index HI = 0.36.

Figure 6 can be used to estimate the statistical precision to which MC| can be measured using the apparatus shown in Fig. 1. As an example, for a fluid with HI = 0.4 and a count time of 20 minutes, the statistical standard deviation of the Mc, measurement is ±27 PTB. Precision in terms of percent water cut in an oil-water liquid phase can be estimated using the right hand side of Fig. 6. As an example, for a 45 fluid with HI — 0.6, a salt water phase salinity of 175,000 ppm NaCI, and a count time of 25 minutes, the percent water cut of the liquid phase can be measured to ±0.02%.

It should be noted that the chart in Fig. 6 represents maximum precisions that can be expected. Actual precision in a homogeneous fluid would be degraded 10 to 20% by systematic errors in the gamma ray detection equipment. In addition, the precision would be degraded if the measurements 50 were made through steel rather than fiberglass pipe.

To determine the test on non-homogeneous gas mixing further tests were performed in the fluid with the acrylic resin tubes geometrically grouped adjacent, diametrically opposite and at a 90° angle (quadrature) within the pipe 10 with respect to the apparatus A on the periphery on the pipe 10. The geometry, hydrogen index (HI) of the fluid, salinity of the liquid phase, and corresponding symbols are 55 tabulated in the chart below.

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GB 2 071 312 A 4

SYMBOL

GEOMETRY

CHART I HI

Mc, (PTB)

TUBE FLUID

1a

ADJACENT

0.92

24

AIR

2a

QUADRATURE

0.92

24

AIR

3a

OPPOSITE

0.92

24

AIR

1b

ADJACENT

0.82

23

AIR

2b

QUADRATURE

0.82

23

AIR

3b

OPPOSITE

0.82

23

AIR

1c

ADJACENT

0.71

22

AIR

2c

QUADRATURE

0.71

22

AIR

3c

OPPOSITE

0.71

22

AIR

1d

ADJACENT

0.35

19

WATER

2d

QUADRATURE

0.35

19

WATER

3d

OPPOSITE

0.35

19

WATER

1e

ADJACENT

0.22

19

WATER

2e

QUADRATURE

0.22

19

WATER

3e

OPPOSITE

0.22

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WATER

1f

ADJACENT

0.82

945

AIR

2f

QUADRATURE

0.82

945

AIR

3f

OPPOSITE

0.82

945

AIR

The values of Rc, are plotted as a function of CH in Fig. 9 for each nonhomogeneous fluid. The solid curves in Fig. 9 are constant salinity curves, Ma, and constant hydrogen index curves, HI, obtained from least-squares fitting data in the homogeneous fluids of Fig. 3. It can be seen from Fig. 9 that, within the 5 limits of experimental error, the data points fall along the proper MCj curves. This indicates that, even 5

though the mixture is nonhomogeneous, an accurate salinity measurement can be made with the present invention.

The fluid hydrogen index HI read from Fig. 9 is a function of the position of the source-detector assembly relative to the location of the fluid inhomogeneity. If we define Hlj as the hydrogen index 10 measured with the source-detector assembly at position j = 1 (for Adjacent), 2 (for Quadrature) or 3 (for 10 Opposite), it can be determined that

(HI, + 2HI2 + HI3)/4 (2)

closely represents the true hydrogen index of the nonhomogeneous fluid. This indicates that if the gas and liquid phases of nonhomogeneous fluids "wander" randomly within the flow line with a frequency 15 much shorter than the count time, then the "average" measured hydrogen index would closely 15

represent the true hydrogen index of the fluid. This criterion can be met in many oil field situations by choosing an advantageous location (such as a riser) on which to install the apparatus A.

Additional chlorine capture gamma radiation occurs within the energy region 5.00 to 5.75 MeV. The statistical precision of the Mc, measurement can, therefore be improved by reducing the low bias of 20 the chlorine window from 5.75 MeV to 5.00 MeV (Fig. 2B). As discussed in copending U.S. Patent 20

Application Serial No. 920,568, sulfur produces interfering radiation at 5.41 MeV which would,

however, be included in such an enlarged chlorine window. For a chlorine window of 5.00 to 8.00 MeV, a 1 % variation in sulfur content of the liquid phase appears as a 37 PTB variation in the Mc,

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measurement. If, however, the sulfur content of the crude is known and remains relatively constant, the apparatus A can be calibrated for a specific sulfur concentration.

Fig. 7 shows a plot of R^, as a function of CH where R^, is defined as the ratio of count rate recorded in energy region 5.00 to 8.00 MeV to the hydrogen count rate CH. Each data point represents a 5 total count time of 25 minutes with the numbers in brackets again indicating Mc, and HI, respectively. 5 Error propagation calculations show that the statistical precision read from Fig. 6 are improved an additional 21% using the larger chlorine window of Fig. 2B.

It should be noted that, in obtaining the salt content of the liquid phase of the fluid, the hydrogen index, HI, of the fluid is also obtained. If the gas and liquid phases of the fluid are moving past the 10 apparatus A at the same velocity and if the following additional information is available or is monitored: 10

(1) line temperature

(2) line pressure

(3) hydrogen index of the liquid phase

(4) hydrogen index of the gas phase

15 the gas/liquid ratio can be computed. 15

It should also be noted that the sulfur content of the liquid phase, Ms, can be determined simultaneously with the salt content MC| and HI by determining, in accordance with copending U.S. Application Serial No. 872,981, an additional ratio

COUNT RATE RECORDED IN ENERGY WINDOW 3 (Fig. 2A)

Rs=

COUNT RATE RECORDED IN ENERGY WINDOW 2 (Fig. 2A)

20 This ratio, along with Rc, and CH are entered into a plot shown in Fig. 8 to determine Mc!, Ms and HI. 20

The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction may be made without departing from the spirit of the invention.

Claims

25 1 ■ A method for analysis of a multiphase fluid containing liquid and free gas and flowing in a 25

conduit, comprising the steps of:

(a) bombarding the fluid with fast neutrons which are slowed down and thereafter engage in thermal neutron capture reactions with materials in the fluid;

(b) obtaining gamma ray energy spectra of the materials in response to the capture of thermal

30 neutrons by the materials in the fluid; 30

(c) obtaining a measure of the concentration of hydrogen in the fluid from the gamma ray energy spectra;

(d) obtaining from the measure of hydrogen concentration the hydrogen index of the fluid;

(e) obtaining a measure of the concentration of chlorine in the fluid from the gamma ray energy

35 spectra; 35

(f) obtaining a ratio of the concentration of chlorine to the concentration of hydrogen; and

(g) obtaining from the concentration ratio of chlorine to hydrogen and the hydrogen index of the fluid a measure of the common salt content of the fluid.

2. A method according to Claim 1, wherein said step of obtaining gamma ray energy spectra

40 includes counting gamma rays in the energy range of from 2.05 MeV to 2.50 MeV. 40

3. A method according to Claim 1 or Claim 2, wherein said step of obtaining gamma ray energy spectra includes counting gamma rays in the energy range of from 5.75 MeV to 8.0 MeV.

4. A method according to Claim 1 or Claim 2 wherein said step of obtaining gamma ray energy spectra includes counting gamma rays in the energy range of from 5.00 MeV to 8.0 MeV.

45 5. A method according to Claim 1 including the further step of obtaining a measure of the 45

concentration of sulfur in the fluid from the gamma ray spectra.

6. Apparatus for analyzing a multiphase fluid containing liquid and free gas and flowing in a conduit, comprising:

(a) means for bombarding the fluid with fast neutrons which are slowed down and thereafter

50 engage in thermal neutron capture reactions with materials in the fluid; 50

(b) means for obtaining gamma ray energy spectra of the materials in response to the capture of thermal neutrons by the materials in the fluid;

(c) means for obtaining a measure of the concentration of hydrogen in the fluid from the gamma ray energy spectra;

55 (d) means for obtaining from the measure of hydrogen concentration the hydrogen index of the 55

fluid;

(e) means for obtaining a measure of the concentration of chlorine in the fluid from the gamma ray energy spectra;

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GB 2 071 312 A 6

(f) means for obtaining a ratio of the concentration of chlorine to the concentration of hydrogen;

and

(g) means for obtaining from the concentration ratio of chlorine to hydrogen and the hydrogen index of the fluid a measure of the common salt content of the fluid.

5 7. Apparatus according to Claim 6, wherein said means for obtaining gamma ray energy spectra 5 includes means for counting gamma rays in the energy ranges of from 2.05 MeV to 2.50 MeV and from 5.75 MeV to 8.0 MeV.

8. Apparatus according to Claim 6, wherein said means for obtaining gamma ray energy spectra includes means for counting gamma rays in the energy ranges of from 2.05 MeV to 2.50 MeV and from

10 5.00 MeV to 8.0 MeV. 10

9. Apparatus according to any one of Claims 6 to 8, wherein said spectra obtaining means includes a gamma ray detector spaced apart from said bombarding means, and in that shielding material is located in position substantially to fill the space directly between the detector and the bombarding means.

15 10. Apparatus according to Claim 9, wherein said detector and said bombarding means are 15

mounted on an outer surface of a said conduit on an axis generally parallel to the axis of the conduit.

11. A method according to Claim 1 and substantially as described herein with reference to the accompanying drawings.

12. Apparatus for analyzing a multiphase fluid containing liquid and free gas and flowing in a

20 conduit, substantially as described herein with reference to the accompanying drawings. 20

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1981. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.